What factor significantly contributes to uterine atony?

Uterine atony refers to the lack of muscle tone in the uterus, which can lead to postpartum hemorrhage. Multiple gestations significantly contribute to this condition because the uterus is overstretched due to the presence of more than one fetus. This overstretching can impair the uterus's ability to contract effectively after delivery. The more the uterine muscle fibers are stretched, the less efficient they become at contracting, particularly if the uterus has been extremely distended.

While prolonged labor and administration of epidural anesthesia may play roles in the overall context of labor and delivery, they are not as directly linked to the mechanical influences on uterine tone as the factor of multiple gestations. Excessive weight gain is more associated with potential complications during pregnancy but does not directly affect the uterine muscle tone in the same manner as how carrying multiple fetuses does.